Mailgun sends "Unknown" for city/country when it cannot geolocate a
recipient, which does not fit the CHAR(2) country_code column and made
the webhook fail. Normalize the mapped country_code for every driver and
store it only when it is an ISO 3166-1 alpha-2 code.

Fixes backstagephp/laravel-mails#92

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>